1. Technical Field
This invention relates to video display systems and, more particularly, to an arrangement for improving the resolution of such systems.
2. Description of the Prior Art
Various information display systems have been proposed and used heretofore employing interactive computer systems wherein the user manipulates the information stored in or generated by a computer. This information may constitute text, graphics, facsimile, video and the like. In the recent past, the advent of very large scale integrated circuitry has made possible the wide use of low cost microprocessor systems. It is now considered cost effective to maintain central databases from which information can be accessed with such a system by the general public.
Two information display systems using central databases, viewdata and teletext, are presently being introduced in the telecommunications industry and have a potential for wide use. In the viewdata system, the consumer is provided a two-way interactive service capable of displaying pages of text and pictorial materials on a video display. In the teletext system, the consumer is provided with a one-way broadcast information service for displaying pages of text and graphic material on a video display.
In both the viewdata and teletext systems, it is necessary to include some type of electronics module at the consumer end with the display control information. One example of such a module is shown in U.S. Pat. No. 4,396,989, issued to J. R. Fleming et al. on Aug. 2, 1983, and is incorporated herein by reference. This type of module provides control to the consumer's display terminal (such as an ordinary television set) for assembling and displaying text and graphic information.
A common approach for displaying such text and graphic information is to use video display circuitry with a local video memory commonly referred to as a bit plane memory or full frame buffer. In that typical display terminals are configured to be refreshed on the order of 60 times per second, the local memory containing a description of the image can be maintained by external circuitry conveniently located in the electronic module. This avoids the need to rebroadcast the content of each screen full of information continuously, which, incidentally, requires very high bandwidth.
In presently existing viewdata and teletext systems utilizing a full frame buffer, the video memory is normally designed to have a given number N of bits per picture element (PEL) and hence be capable of displaying 2.sup.N colors. If a display of H horizontal PELs by V vertical PELs is desired, a video memory of N.times.H.times.V bits would be required. And if a display having a horizontal resolution of 2H is desired, twice as many bits would be required in the video memory.
The horizontal resolution of existing display devices is not easily expanded since the frequency of the horizontal scan is limited. This has led designers to constrain the number of horizontal PELs for display on a home television receiver to approximately 256, even though the display tube is capable of greater resolution. It is desirable, therefore, to be able to increase the horizontal resolution of a display device and thereby more fully utilize its resolution capability, but at the same time avoid the normal corresponding increases in horizontal frequency scan and video memory requirement.